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Why is Hotmail blocking email from Office 365?

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Diagnostic information for administrators:

Generating server:

Remote Server returned '550 OU-001 (COL004-MC1F11) Unfortunately, messages from weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to'

--- shows the blocked SMTP server to be in the Microsoft London Internet Data Center.


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  • This is not only, companies who use filters that checks blockings in database can't receive mails from Office 365 users.

    This may be a major problem for many Office 365 customers, I have reported the problem to the O365 support.

  • Hi All,

    Sorry for the inconvenience this case has caused. We have been aware of the issue, and have reported this case to the related team.

    Since around 10am BST we have started to receive new cases on EOP IP being listed in the Spamhaus CBL list. This is affecting outbound mail to some recipients (i.e. where the EOP sending IP is and the recipient org uses Spamhaus for anti-spam protection).

    Although the IP had already been removed from the CBL list, there may be a little delay for the outbound emails. We are actively working on the resolution and would appreciate your patience.  

    Thanks for your understanding.
    Best Regards,
    Jason Jiang

    2 out of 2 people found this post helpful.